1. Field of the Invention
The present invention relates to a pressure-regulating device for containers used for storage, transportation and disposal of dangerous substances such as low- and medium-level radioactive wastes and industrial wastes.
2. Description of the Prior Art
With the continuous increase in the amounts of such wastes (1) various radioactive wastes generated from nuclear power plants and other nuclear facilities and (2) harmful heavy metal sludges issued from chemical plants, operators and researchers are making every effort to develop safe and economical ways to store, transport and dispose of these wastes.
Radioactive substances differ from heavy metals in that individual nuclides have their own half-lives and need to be isolated from the biosphere for limited periods. In the current nuclear fuel cycle that involves nuclear fission, most of the long-lived wastes originate from the spent fuel reprocessing plants. Beta- and gamma-emitting radioisotopes such as .sup.90 Sr and .sup.137 Cs have half-lives of several hundred years, and alpha-emitting transuranics having atomic numbers of 93 or more have estimated half-lives of hundreds of thousands of years. These radioisotopes are typically discharged as high-level radioactive wastes. It is considered that they should first be stored temporarily as liquids, then solidified by suitable methods and stored by utilizing various engineering techniques and finally disposed of. Intermediate- and low-level wastes of low concentration, however, are discharged in far greater amounts than high-level wastes and it is generally understood that their half-lives are not more than about a hundred years. In other words, ideal containers for land storage of low- and intermediate-level radioactive wastes should confine them safely for at least about a hundred years.
Many containers to be used for storage, transportation and disposal of intermediate- and low-level radioactive wastes are currently being or have been proposed.
One of such containers is a high integrity container in actual use wherein a concrete reinforced with steel fiber, wire netting or the like is strongly bonded to the inner surface of a metal container with an impregnant such as a polymer or an inorganic substance (this concrete is hereinafter referred to as SFPIC) hereby the long-term durability and easiness of handling are improved and the reduction of the internal volume is minimized.
Containers used for storage, transportation and disposal of radioactive wastes, industrial wastes, etc. have experienced, during he period of storage, transportation and disposal, problems of container expansion or breakage caused by gas generation due to the chemical reaction of the contents and by the resulting increase in gas pressure inside the container. In order to structurally protect the containers from such problems, it is required that the internal pressure of the container be kept at a positive pressure of 50% or less of the pressure resistance of the container by an appropriate means, that the means has sufficient durability, that the inflow of water into the container through the means be 0.1% or less of the internal volume of the container over 100 hours even when the container is subjected to a hydraulic pressure corresponding to the water head at the depth at which the container is to be buried, and that the means will not break or part company with the container or damage it in any way even in the event that the container is dropped due to an accident.